Black Holes Don't Exist?

As blumfeld0 has already said, a topic (first posted by SF) that has come up on another thread deserves a thread of its own.

A controversial, just-published paper which asserts that black holes never form is generating substantial interest within the astrophysics community.

The standard view is that after a supernova explosion, the core of a sufficiently massive star collapses and forms a black hole. Because of inifinite gravitational time dilation, an observer who hovers above the collapsing core never see a black hole form, but an observer that who sits on the "surface" of the collapsing core rapidly finds herself inside a black hole. Also, an astronaut who takes a trip towards such a collapsing core can cross the event horizon of a black hole in a finite amount of proper time.

This new paper says that, due to Hawking radiation, NO observer finds himself inside the event horizon of a black hole. I expect the dust will settle on this issue within a couple of years, but things might be settled: in a substantially shorter period of time; in a substantially longer period of time, if a fully quantum theory of gravity is needed to decide what happens.

"Instead it may happen that the true event horizon
never forms in a gravitational collapse. We saw that an
outside observer never sees formation of a horizon in fi-
nite time, not even in the full quantum treatment. What
about an infalling observer?......The infalling observer never
crosses an event horizon, not because it takes an infinite
time, but because there is no event horizon to cross. As
the infalling observer gets closer to the collapsing wall,
the wall shrinks due to radiation back-reaction, evaporating before an event horizon can form. The evaporation appears mysterious to the infalling observer since his detectors don’t register any emission from the collapsing wall. Yet he reconciles the absence of radiation with the evaporation as being due to a limitation of the frequency range of his detectors. Both he and the asymptotic observer would then agree that the spacetime diagram for an evaporating black hole is as shown in Fig. 9. In this picture a global event horizon and singularity never form. A trapped surface (from within which light cannot escape) may exist temporarily, but after all of the mass is radiated, the trapped surface disappears and light gets released to infinity."

Here is the link to a subsequent, unpublished http://www.arxiv.org/PS_cache/gr-qc/pdf/0701/0701096v1.pdf" [Broken].

From this article: "Due to the backreaction of evaporation, the Schwarzschild radius, R S , must decrease with time. On the other hand, as the collapse proceeds, the gravitational time delay from the surface of the shell to the asymptotic observer becomes larger and would become infinite if the shell were to ever cross its (instantaneous) Schwarzschild radius. So no signal from the collapsing shell can arrive to the asymptotic observer so as to convey that a black hole has formed and neither can any object be seen to disappear into a black hole at any finite time. But the asymptotic observer does receive the full energy of the infalling matter in a finite time, given by the evaporation time. The only logical conclusion is that the collapsing object evaporates before a black hole can form."

There is the question that as matter collapses into a BH and an event horizon forms around it then an external observer will not see the final stages of that formation as photons coming from the material would be 'frozen' by time dilation (infinite red shift) at the horizon itself.

This does not mean the BH 'singularity' has not formed internal to that event horizon - just that it cannot be seen, which is what is meant by a black hole in the first place.

In particular, the article states that there are no singularities or event horizons on a global spacetime diagram that represents the complete history of the collapsing object. This is spectacularly different than the global spacetime diagrams for collapsing objects that appear in, for example, Wald.

Well the idea that Hawking radiation means that nothing can actually fall into a black hole (so that it can't form in the first place), is sufficiently widespread to have answer in Ted Bunn's Black Holes FAQ. In Black Holes - do they exist? I argue that this answer is fallacious, as it uses the past tense for events which are not in an observers past light cone - that is it uses an 'everyday' concept of time, which just isn't valid in this context, as does.

I have a problem ... with existing observations of potential BHs that would be difficult to explain another way.

I don't think that these are a problem -what we have observed is masses sufficiently large that current theory says that they must form a black hole. Any alternative theory which relies on Hawking radiation is unlikely to have any observable effects for macroscopic black holes - the matter just piles in and eventually comes out as Hawking radiation as it would for current theory. It's only if we can create microscopic black holes that such a theory could be tested.

I can appreciate that the event horizon is always in my future and so the singularity is arguably an abstraction. I can also appreciate that I can never fall through the event horizon in co-ordinate time because of that infinite time dilation. That doesn't make me think that black holes don't exist, just that the interpretation of what they are needs some further thought. Maybe I don't understand them, but I think I can. But what I can't understand is Hawking Radiation. Can somebody explain it to me?

I've never seen a virtual particle, and I never will. Real particles are virtual enough, be they fermions or bosons, and I cannot conceive of anything that has a "runaway repulsion" negative mass that can get past the infinite time dilation to "shrink" the hole. When I look at the electron and its antiparticle the positron, I see that both are made out of 511keV of positive energy. If these are being created as virtual pairs and we lose one to the black hole, the black hole would grow, not shrink. That's a big bang scenario, not evaporation. If we're talking about the photon that is its own antiparticle, I still struggle, because every photon has energy/momentum, and things are still the wrong way round. I'm left struggling, trying to understand virtual particles, and wondering whether they are just an abstraction, accounting units for a mathematics that doesn't properly grapple with QFT. It feels like we've got abstraction fighting another, like the invisible man wrestling the negative carpet, and I just can't make sense of it.

Edit: I was looking for an arxiv paper that I've got at home arguing that Hawking Radiation is rather too much a given, but can't find it. For what it's worth, I did see this:

However, not everyone is convinced. Nobel laureate Gerard 't Hooft of Utrecht University in the Netherlands said the process can in no way produce enough radiation to make a black hole disappear as quickly as is being suggested.

“The horizon forms long before the hole can evaporate,” New Scientist quoted 't Hooft as saying. The study appears in the journal Physical Review D.

I can appreciate that the event horizon is always in my future and so the singularity is arguably an abstraction. I can also appreciate that I can never fall through the event horizon in co-ordinate time because of that infinite time dilation. That doesn't make me think that black holes don't exist, just that the interpretation of what they are needs some further thought. Maybe I don't understand them, but I think I can.

From my comments on the Bad Astronomy Blog

The main (and controversial) point of the paper is that *no* observer (hovering, freely falling, blasting away with rocket *towards* the surface of the collapsing object, etc.) experiences an event horizon because there is no event horizon to experience.

This contasts with the standard view: stationary observers hovering above the collapsing object never see the event horizon form, but an observer sitting on the surface of the black hole ends up inside an event horizon after a finite amount of proper time, and a freely falling observer also can cross this event horizon in a finite amount of proper time.

The global, observer-independent Penrose diagrams of all of spacetime in Figures 8 and 9 of the paper illustrate well the differences between the standard view and the paper’s view. Figure 8 (same as page 419 of Carroll’s text) has an event horizon and a singularity; Figure 9 has no event horizon and no singularity. Since these are global, observer-independent diagrams, the paper puts forth the view that no observer see an event horizon or a singularity.

But what I can't understand is Hawking Radiation. Can somebody explain it to me?